Capacitive position sensor

ABSTRACT

A capacitive rotor position sensor generates three phase output pulses indicative of the position of the rotor of a three phase commutatorless D.C. motor. A radially elongated capacitor plate connected to the motor rotor is rotatably mounted adjacent 12 circumferentially displaced stationary capacitor plates, and a high frequency oscillator is coupled to the rotatable plate. A differential amplifier for each phase has one input coupled to three adjacent first stationary capacitor plates which together subtend 180 electrical degrees and the other input coupled to three adjacent second stationary capacitor plates which together subtend 180 electrical degrees and are displaced 180 electrical degrees from the first plates, and the corresponding inputs of the three differential amplifiers are coupled to first stationary plates displaced 120 electrical degrees apart and to second stationary plates displaced 120 electrical degrees apart. The high frequency pulses from the oscillator are coupled through the rotatable plate to the stationary plates, and the differential amplifiers enhance the one-to-zero ratio of (1) a logic one signal from a stationary plate opposite the rotatable plate at a given instant; and (2) a logic zero signal from a stationary plate simultaneously displaced 180 electrical degrees from the rotatable plate, to derive square wave output pulses at a frequency proportional to motor speed indicative of the rotor position.

United States Patent 1 Stich [4 1 Sept, M, 1973 CAPAClTlVE POSITIONSENSOR Frederick A. Stich, Hales Corners,

Wis.

[73] Assignee: Allis-Chalmers Corporation,

Milwaukee, Wis.

[22] Filed: May 15, 1972 [21] Appl. No.: 253,418

[75] inventor:

[52] [1.8. CI 340/200, 318/138, 318/254, 5 317/249, 323/93 [51] Int. Cl.H021) 1/00- [58] Field of Search 340/200; 318/254; 317/249; 323/93 [56]q References Cited UNITED STATES PATENTS 2,980,839 4/1961 Haessermann318/138 3,214,663 10/1965 Kreutzer 318/254 3,346,792 10/ l 967 Noumi318/138 Primary Examiner-John W. Caldwell Assistant Examiner-Robert J.Mooney Attorney-Lee H. Kaiser et al.

[57] ABSTRACT A capacitive rotor position sensor generates three phaseoutput pulses indicative of the position of the rotor of a three phasecommutatorless DC. motor. A radially elongated capacitor plate connectedto the v motor rotor isrotatably mounted adjacent l2 circumferentiallydisplaced stationary capacitor plates, and a high frequency oscillatoris coupled to the rotatable plate. A differential amplifier for eachphase has one input coupled to three adjacent first stationary capacitorplates which together subtend 180 electrical degrees and the other inputcoupled to three adjacent second stationary capacitor plates whichtogether subtend 180 electrical degrees and are displaced 180 electricaldegrees from the first plates, and the corresponding inputs of the,three differential amplifiers are coupled to first stationary platesdisplaced 120 electrical degrees apart and to second stationary platesdisplaced 120 electrical degrees apart. The high frequency pulses fromthe oscillator are coupled through the rotatable plate to the stationaryplates, and the differential amplifiers enhance the one-to-zero ratio of(1) a logic one signal from a stationary plate opposite the rotatableplate at a given instant; and (2) a logic zero signal from a stationaryplate simultaneously displaced 1 80 electrical degrees from therotatable plate, to derive square wave output pulses at a frequencyproportional to motor speed indicative of the rotor position. 1

25 Claims, 6 Drawing Figures Patented Sept. 18, 1973 3 Sheets-Sheet 1Patented Sept. 18; 1973 3 Sheets-Sheet 2 Patented Sept. 18, 1973 3Sheets-Sheet 5 CAPACITIVE POSITION SENSOR This invention relates tocommutatorless D.C. motors and in particular to capacitive transducersfor detecting the position of the rotor of a commutatorless D.C. motor.

BACKGROUND or THE INVENTION Commutation in a conventional D.C. motor isessentially a mechanical switching operation in which brushes and, asegmented commutator cyclically reverse currents through the armatureconductors in a sequence as a function of rotor position, and suchcommutation results in friction wear and sparking with the attendantgeneration of radio frequency noise. In order to eliminate such defects,commutatorless D.C. motors have been developed provided with electroniccommutation means for controlling the armature current in accordancewith the rotational position of the rotor.

Brushless D. C. motors are also known which employ a permanentlymagnetized rotor and wherein the stator windings are energized in acyclical sequence through electronic switching means which aresequentially gated in accordance with the rotational position of therotor. A

Some known rotor position sensors modulate a source of radiant energy,while other known-rotor position sensors use a Hall element or employ apermanent magnet in the periphery of the rotor to induce an e.m.f. in acontrol winding. Such prior art rotor position detectors, in general,have low sensitivity and relatively high temperature drift, and suchfactors require that the associated circuitry be excessively complicatedand expensive in order to obtain the desired accuracy, reliability andlow maintenance. Capacitive rotor position DESCRIPTION OF THE DRAWINGSphase square wave output from the capacitive position transducer of FIG.l; and (b) the train of pulses from the synchronous pulse generatorwhose frequency. is a function of motor speed.

sensors have also been employed with brushless D.C.

motors in an effort to reduce expense and'to minimize maintenance, butthe accuracyof known capacitive transducers for measuring angularposition is limited by capacitive fringing effects and is dependent uponrotor velocity. r

OBJECTS OF THE INVENTION It is an object of the. invention to provide animproved capacitive position sensor for indicating the position of arotatable member.

It is another object of the invention to provide an im- Y pensive yethighly accurate-and-reliable.

A further object of the invention is to provide such a capacitive rotorposition detector for the electronic commutation of a D.C. motor whichis not velocity dependent and provides an accurate indication of rotorposition even at stall. I

. A still furtherobject-of the invention is to provide a capacitiverotor positionsensor which has high sensitivity and low temperaturedrift and provides a polyphase pulse output at *8 frequency proportionalto motor speed adapted to control electronic switches for energizing thestator windings of a polyphase electric motor in a direction andcyclical sequence which will maintain the rotorfield locked in with therotating stator field of. the motor. v

SUMMARY OF THE INVENTION A capacitive position sensor in accordance withthe invention generates output pulses indicative of the position of arotatable member and has a radially elongated capacitor plateoperatively connected to the rotatable member and rotatably mountedadjacent a plurality of circumferentially displaced stationary capacitorplates. A high frequency oscillator is coupled to the rotatable plate,and means coupled to the stationary capacitor plates and responsive .tothe pulses from the oscillator flowing through the capacitance betweenrotatable and stationary plates generate an output pulse when therotatable plate is opposite each stationary plate.

The preferred embodiment indicates the position of the rotor of a motorhaving n phases and p rotor pole pairs. 2pn stationary capacitor platesare circumferentially displaced apart in a plane opposite a rotatable capacitor plate having one radial arm for each rotor pole pair. Adifferential amplifier for each phase has one 'input coupled to aplurality of adjacent first stationary capacitor plates which togethersubtend electrical degrees to generate a phase output signal and itsother input coupled to a plurality of adjacent second stationary plateswhich togethersubtend 180 electrical degrees and are displaced 180electrical degrees from the first stationary plates to generate thecomplement of the phase output signal. The corresponding stationaryplates coupled to the n stationary amplifiers are displaced 360/nelectrical degrees so that the output is an ri-phase train of pulseshaving a frequency proportional to the angular velocity of the motorrotor. The differential amplifiers enhance the one-to-zero ratio of: (l)a logic one voltage signal from a stationary plate which the rotatableplate is opposite at a given instant; and (2). a logic zero voltagesignal from, a stationary plate simultaneously displaced 180 electricalfrom the rotatable plate. Bistable circuit means provide positivefeedback to the inputs of the differential amplifiers to enhancediscrimination between stationary capacitor plates.

v DETAILED DESCRIPTION Referring to the drawing, the capacitiverotor-position sensor 11 of the preferred embodiment shown in FIGS. l-4determines the instantaneous position of a commutatorless motor rotor I3and provides a threephase square wave output A, A, B, B, C and C shownin FIG. 5a at a frequency proportional to motor speed and which isindicative of rotor position and thus of the position of the rotor fieldwith respect to the motor stator windings (not shown). The three-phaseposition sensor output establishes the correct stator field sequencewith respect to the rotor R so that the power transistors or SCRs whichenergize the stator windings conduct at the right time and in the propersequence to maintain the desired direction and rotational velocity ofthe stator rotating field.

Capacitive rotor position sensor 11 is particularly adapted for use in amotor control system disclosed in the copending US. patent applicationof Frederick A. Stich'and Allois F. Geiersbach, S.N. 266,286 entitledControl ForCommutatorless Motor filed June 26, 1972, and to. control thepower transistors which energize the stator windings of a commutatorlessDC. motor disclosed in the copending US. Patent application ofFrederickA. Stich and Glenn W. Schwantes, S.N. 278,577 entitledCommutatorless Motor filed August 7, 1972 both of which have the sameassignee as this invention. v

The motor rotor has p pole pairs, where p is an integer, and thecapacitor rotor position sensor has at least pv times a arcuatelydisplaced stationary capacitor plates, where n is an integer equal tothe number of motor phases. Motor rotor R of the preferredembodiment'shown inFIGS. 14 has 2p pole pairs, i.e., four poles, andcapacitive rotor position sensor 11 of the preferred embodiment may have2pn equals 12 circumferentially displaced metallic capacitor plates, orelectrodes- 31 in a common plane perpendicular to the motor rotor axisand defining an annular disk. The stationary electrodes 31 may be copperplates in the shape of a sector of a ring subtending an arc of 360/2nequals 60 electrical degrees and affixed to a stationary-annular statorboard 40 (See FIGS. 2 and3) mounted by fastening means 4l'to the end,bell 42 of the brushless motor frame S. Stationary stator board 40 maybe of an insulating material such as fibre glass impregnated with epoxyresin.

The capacitive rotor position sensor of the invention has a'radiallyextending rotatable capacitor plate for each pole pair with the radiallyextending arms displaced 360 electrical degrees. In the preferredembodimeat having two rotor pole pairs, a diametrically elongated,rotatable capacitor plate 35 operatively connected to the motor rotor Rfor rotation therewith and mounted for rotation adjacent stationarycapacitor plates 31 comprises two electrically commoned, elongated andnarrow radially extending electrodes 36 aligned along adiameter. The twoelectrodes 36 which constitute rotatable capacitor plate 35 areschematically shown in FIG. 1 as electrically commoned and connected bya conductor 37 to a movable electrode- 38 of a coupling capacitor'Clhavingits stationary electrode 39 coupled to the output of a relaxationoscillator which is capable of generating fast-rise, or steep wavefrontpulses.

Stationary electrode 39 of coupling capacitor C1 may be a thin annularcopper ring 39 (See FIGS. 2 and 3) affixed to stationary stator board 40radially inward from the stationary capacitor plates 31. Movableelectrode 38 of coupling capacitor C1 may be a thin annular coppermember affixed to a rotating annular rotor board 43 which is fastened tothe motor rotor R and preferably is of an insulating material such asfiber glass impregnated with epoxy resin. Movable electrode 38 ofcoupling capacitor C1 so affixed to rotatable rotor board 43 may bespaced by an air gap from stationary electrode 39 fastened to stationaryrotor board 40. Movable electrode 38 of coupling capacitor C1 may havethin, diametrically opposed fingers 36 integral therewith extendingradially outward which constitute the two radially elongated electrodes36 that define rotatable capacitive plate 35. Radial conductors 33electrically connected to ground are disposed between adjacentstationary capacitor plates 31, and a grounding conductor ring 35 may bedisposed between the outer periphery of the stationary electrode 39 ofcoupling capacitor C1 and the inner margin of the stationary electrodes31 for the purpose of electrically isolating the stationary plates 31from each other so that discrimination between the plates is high.

RELAXATION OSCILLATOR Relaxation oscillator 10 may include a capacitorC2 charged from the battery BAT through a resistance R31 and coupledthrough a diode D11 to the emitter of a transistor Q21. The potential onthe base of transistor Q21 is established by a-voltage dividercomprising three resistors R32, R33, and R34 connected in series acrossthe battery terminals. When the voltage on capacitor C2 rises to apredetermined potential, the emitter of transistor Q21 becomes forwardbiased, and transistor Q21 turns on and discharges capacitor C2 throughits collector resistor R35 to the positive terminal of the battery,thereby applying the voltage drop across resistor R35 to the base of atransistor Q22 and turning it on. Conduction by transistor Q22 lowersthe voltage on its'collector and generates an output pulse which iscoupled through a resistor R36, a diode D12, and the fixed and rotatingplates 39, 38, of coupling capacitor C1 to the rotatable capacitor plate35 of capacitive position sensor 11. Such charging of capacitor C2 anddischarging thereof through transistor Q21 and resistor R35 is repeatedrapidly to generate output pulses from relaxation oscillator 10 at ahigh frequency.

In order to obtain high accuracy, oscillator 10 has a frequency which ishigh relative to the electrical frequency that is synchronous to themotor, for example, 20 KHZ for a 5,000 RPM, 4-pole motor. The highfrequency pulses generated by relaxation oscillator 10 are coupledthrough rotatable capacitor electrode 35 to the stationary capacitorplates 31 and the load connected thereto. Oscillator 10 thus derivesread-out" pulses with high frequency components which are distributed byrotatable capacitor plate 35 to a square wave generator 12 through thefixed capacitor plates 31'arranged in the sequence shown in FIG. 1. Thehigh frequency components of the read-out'pulses readily couple throughadjacent rotatable and fixed capacitor plates 35 and 31. The rotatableelectrodes 36 (which define rotatable-capacitor plate 35) and the fixedplates 31 are preferably elongated in a radial direction to obtainadequate capacitance, but the width of the rotatable electrodes 36 isheld to a minimum to obtain high accuracy. Inasmuch as the preferredembodiment is for a four-pole rotor R, a set of output pulses A, A, B,13,

C and C shown in FIG. 5a will be generated during (mechanical) ofrotation of rotor R, and consequently, diametrically opposite fixedplates 31 which contain the same 7 information are electricallyconnected together. The read-out pulses received on the stationaryplates 31a-31f are converted by the square wave genervidual couplingresistors RA1 across a common load resistor LRA. The output signal froma stationary plate 31 developed across a load resistor LR when rotatableplate 35 is opposite thereto may be considered a logical one voltage,while the output signal from a stationary plate 31 when rotatable plate35 is remote therefrom may be considered logical zero voltage. Theindividual stationary plates 31 subtend an arc of approximately 360/2nequals 60 electrical degrees, and the three adjacent stationarycapacitor plates 31a, 31b, 31c for phase A coupled to load resistor LRAtogether subtend 180 electrical degrees (so that the phase A outputpulse is of 180 electrical degrees duration) and are also displaced360/n equals 120 electrical degrees (60 mechanical) from" the threecorresponding adjacent stationary plates 31c, 31d, and 31e associatedwith phase B and also 120 electrical degrees from the threecorresponding adjacent stationary capacitor plates 31e, 31f, and 31aassociated with phaseC.

The output fromrelaxation oscillator is connected through. diodeDlZacross a 220K resistor R38-to assure that dischargeof thestationarycapacitor plates 31 is not rapid enough to reset the flip-flops whichareemployed in some embodiments to convert the logic zero voltage andlogic one voltage output signalsfrom the stationary capacitor plates 311 into the three phase square waves shown inFIG. 5a.

SQUARE WAVE GENERATOR Square wave generator 12- may include threesimilar differential amplif ers 61A, 61B and 61C each of which isassociated with one of the motor phases A, B and C I and produces theoutput forthat phase. The difierential amplifiers 61A, 61B, and 61Cenhance the one-tozero: ratio ofthe inputs thereto from the stationaryplates developed-across the load resistor LR, and one input toeachdifferential amplifier is a logic one signal from a stationary plate31 having relativelyhigh couplirig at a given instant to rotatableplate35iand the other inputthereto is a logic zero signal from astationary plate having relatively'low coupling at that instant torotatable capacitor plate 35. The two inputs of each differentialamplifier fora given phase are coupled to stationary plates 31 for thatphase displaced 180 (electrical); apart so that the twoinputs arereceiving logic one and logic zero signals from stationary plates 31.

having relatively high and relatively low coupling respectivelywith-rotatable capacitor plate 35. For example, the A and K'inputs todifferential amplifier 61A are respectively coupled throughresistors RA]and RAZ to stationary plates 31a and 31d (or plates 31b and 31d,-

or plates 31c and 31f) designated A whenboth electrodes 36 are oppositestationary plates 31a and rotatable electrode 35 is displaced ;90mechanical degrees (180 electrical).from stationary plate 31d so thatminimum coupling exists 'between'plates 31d and 35.

Each stationary plate 31a, 31b and 310 designated A for phase A subtends30 mechanical degrees electrical), and the three plates 31a, 31b and'31care disposed side-by-side so that together they subtend 180 (electrical)and the A output pulse (which is generated when rotatable plate 35 isopposite plates 31a, 31b and 310 designated A has a period of 180electrical.

The inputs to the differential amplifier 61A, 61B and 61C for each phaseare coupled'to stationary plates 31 displaced (electrical) from thestationary plates 31 to which the differential amplifiers for the othertwo phases are coupled so that the outputsA, B and C are displaced 120-electrical. Stated more broadly, the inputs to the phase differentialamplifiers 61 are coupled to stationary capacitor plates displaced 360/nelectrical degrees, where n is an integer equal to the number of motorphases. For example, the A input to differential amplifier 61A for phaseA is coupled through RAl resistors to stationary plates 31a, 31b, and310, and the B input to differential amplifier 61B for phase B iscoupled through RBI resistors to stationary plates 31c, 31d and 31edesignated 8 which are displaced respectively 120 (electrical) fromstationary plates 31a, 31b and 310 designated A. Similarly, the phase Bplates 31c, 31d and 31a are displaced 120 (electrical) respectively fromthe phase C stationaryplates 31e, 31f and 31a designated C.

The motor rotor R has four poles, and consequently, each pair ofdiametrically opposed plates 31 which are displaced 360 electricaldegrees apart and contain the same information) areelectricallyconnec-ted together and thus increase the capacitivecoupling between rotatable and stationary plates. For example, the pairof diametrically opposed stationary electrodes 31a are connectedtogether, and the pair of diametrically opposed stationary electrodes31b'are electrically connected together; Each pair of electricallycommoned stationary plates 31 'is connected toone of the six in-' putsto each of the three differential amplifiers 61A, 61B and 61C. Forexample, diametrically opposed and electrically. commoned stationaryplates 31a designated A, B, C are connected through a resistor RAl to anA input .of differential amplifier 61A,through a resistor RB2 to a Binput of differential amplifier 61B, and through a resistor RC1 to a Cinput of differential amplifier 61C. The designation of the inputresistors as A resistors, Bresistors, C resistors, etc.,'connotes thatthe particularinput resistor is connected to a pair of diametricallyopposed stationary electrodes 31 bearing this designation and'whichresults in the corresponding output pulse being at logic- 1 voltage whenrotatable electrode 35 is opposite these stationary plates. For example, resistors RA1 are connected to the pairs of stationary electrodes31a, 31b and 310 each of which is designated A and subtends 60electrical degrees and which results in the generation of the A pulseduring ary electrode 31a, 31b, and 31c designated A, and since eachstationary electrode 31c, 31d and 31e subtends 60 (electrical), pulse Bhas a period of 3 X 60 180 (electrical) and is displaced 120(electrical) from the A output pulse.

In alternative embodiments, flip-tlops (not shown) convert the one andzero outputs from the stationary plates 31 to the set of three-phasesquare waves A, A, B, B, C and C shown in FIG. 5a, and preferably suchflip-flops use a common .emitter resistor so that each flip-flopperforms as a positively fed-back difference amplifier during theswitching transition to assure that there is positive discriminationbetween the one and zero inputs.

Square wave generator 12- also preferably includes three NAND gate latchcircuits LCA, LCB and LCC each of which is associated with one of thephases and the differential amplifier for that phase. For example,differential amplifier 61A and latch circuit LCA are associated withphase A and together generate the square wave pulse A for phase A andits negation A (which is the inverse of A).

The three differential amplifiers '61A,'61B and 61C are similar, andonly differential amplifier 61A for phaseA will he described.Differential amplifier 61A is of conventional configuration, and thebase of one transistor Q23 is coupled to load resistor LRA which iscommon to the three resistors RAl which add the signals from thepairs ofposition sensor stationary plates 31a, 31b, and 310 respectively thatare designated A and together subtend 180 electrical degrees. The baseof the other transistor Q24 of differential amplifier 61A is coupled toload resistor m which is common to the three A input-resistors RA2 whichare individually connected to the pairs of stationary plates 31d, 31c,and 31f designat'ed'A and sum the signals therefrom. The active elementsof the differential amplifiers 61A, 61B, and 61C are preferablyintegrated circuits, and differential amplifier 61B is symbolicallyshown as being one half of an integrated circuit which is commerciallyavailable from RCA Corporation under thetype designation CA3026.

Differential amplifier 61A provides a low voltage, or logic 0 output onthe collector ofthat transistor Q23 or Q24 having itsb'ase coupled tothe three pairs of stationaryplates 31a, 315,310 or 31d, 31e, 31],having the greatest coupling to the rotatable electrode 35, therebygiving an indication of the position .of the motor rotor RQFor example,if rotatable electrodes 36 are in a position where thesum of the readoutpulses received on the pairs of'plates 31a, 31b, and 31c designated A isgreater than the sum of. the pulses received on pairs of stationaryplates 31d, 31:, and 31f designated A, transistor 023 will be turned onand the voltage on' its collector will below and transistor 024 will beturned off and its collector potential will be relatively high. Inasmuchas each stationary plate 31 subtends (electrical) the A pulse will havea period of 180 (electrical) while rotatable electrode 35 is oppositethe A st ationary plates'3l a, 31b, and 31c, and similarly, the A pulsewill have a period of 180 (electrical) while rotatable electrode 35 isopposite theA plates 31b, 31c and. 31f. I V

Differential amplifiers 61A,'61B, and 61C enhance the "one-to-zeroratio, of the input from stationary plates 31 and control NAND gatelatch circuits LCA,

LCB, and LCC respectively which convert the enhanced pulses into thesquare waves A, A, B, B, C and C. The collector of transistor Q23 iscoupled to one input of a NAND gate NAND 1, and the collector oftransistor Q24 is coupled to one input of a NAND gate NAND2. The outputfrom gate NANDl is coupled to the other input of gate NAND2 and providespositive feedback to the base of transistor 023 through a resistor R39.Similarly, the output from gate NAND2 is coupled to the other input ofgate NANDl and provides positive feedback through a resistor R40 to thebase of transistor Q24.

Assume the condition where the potential at the collector of transistorQ24 is high and that of transistor Q23 is low. The logic 0 voltage onsaid one input of gate NAND! will provide logic 1 on its output togenerate the A square wave shown in FIG. 5a. The logic 1 output fromgate NANDl is coupled to said other input of gate NAND2 which also haslogic 1 on said one input from the collector of transistor Q24. GateNAND2 thus provides a logic 0 voltage on A output conductor which isalso coupled to the said other input of gate NANDl, thereby latchinggate NANDl with logic 1 on its output and on the A output conductor.

When rotatable electrode 35 is opposite the pairs of stationary plates31d, 31e, and 31 f designated A so that the sumof the readout pulsesfrom relaxation oscillator 10 coupled thereto is greater than the sum ofthe pulses coupled to the pairs of stationary plates 31a, 31b, and 310designated A, transistor Q24-is turned on and transistor Q23 is turnedoff, thereby changing said one input to gate NANDl to logic 1- and saidone input to gate NAND2 .to logic 0 and switching the NAND gate latchcircuit LCA to the opposite state so that logic 1 'voltage appears onthe A lead (as shown at 60 in FIG.

5a) and logic 0 appears on the A output lead.

SYNCI-IRONOUS PULSE GENERATOR Synchronous pulse generator 14 receivesthe threephase square waves A, B, and C and their complements A, B, andC from square wave generator 12 and generates anegative-going pulse Pshown in FIG. 5b at every square wave edge of the outputfrom square wavegenerator 12. Synchronous pulse generator 14 thus derives a train ofpulses wherein each pulse P corresponds to a change of state of thelatch circuits LCA, LCB and LCC. Inasmuch as there are six edges percycle of the three-phase square waves A, A, B, B, C and C, six pulsesare derived by synchronous pulse generator 14 at a frequencyproportional to motor speed for each cycle of the three-phase squarewave output from square wave generator 12.

Synchronous pulse generator 14 includes a singleended differentialamplifier of conventional design wherein the base of one transistor Q25is coupled to the A, Band C leads from the latch circuits LCA, LCB, andLCC of the square wave generator 12 through individual differentiatingcapacitors C3. The base of the other transistor Q26 of differentialamplifier 70 is coupled to the A, B, and C leads from square wavegenerator 12 through individual differentiating capacitors C4. Thecollectors of transistors Q25 and 026 are commoned and connected to theoutput lead in which the train of synchronizing pulses P shown in FIG.5b appears. Each transistor Q25 and Q26 is preferably one half of anintegrated circuit element similar to that sold by RCA Corporation underthe type designation CA3026. The input capacitors C3 and C4differentiate the square wave outputs A, B, C, A, hand 6 from squarewave generator 12, and the differential amplifier 70 is operated in anoverdriven mode and shapes the pulses to form a single train of negativegoing pulses P at the commoned collectors, as shown in FIG. b at afrequency indicative of motor speed and with a pulse P generated at eachchange of state of the NAND gate latch circuits LCA, LCB, and LCC of thesquare wave generator 12, i.e., at every square wave edge of the outputpulses A, A, B, B, C and Pulses P may be delayed in a variable .delaycircuit which controls the duty cycle of power switches (not shown) thatenergize motor stator windings in the desired sequence as disclosed inthe aforementioned copending application of Frederich A.

Stich and Allois F. Geiersbach, Ser. No. 266,286.

The preferred embodiment has been described with adjacent firststationary plates together subtending 180 electrical degrees coupled toone input of each differential amplifier to derive a phase output signal(e.g., stationary plates 31a, 31b and 31c coupled to the A input ofdifferential amplifier 61A) and adjacent second stationary platestogether subtending. 180 electrical degrees anddisplaced 180. electricaldegrees from the first plates coupled to the other inputof thedifferential amplifier to derive the phase signal complement, (e.g.,second stationary plates 31d, 31c, and 31f displaced 1'80 electricaldegrees fromcorresponding first plates 31a, 31b and 310 are coupled tothe A input of differential amplifier-61A to derive signal A), but inalternative embodimentslonly p times n'stationary capacitor plates-areprovided to derive the phase'output signal, e.g.', A, and other. meansareutilized to generate its complement, i.e., X, when the phase outputsignal, such as A, is absent. v v 1 It should-be understood that I donot intend to be limited to the particular embodiment shown anddescribed for many modifications and variations thereof will bereadilyapparent to those skilled in the art.

The embodiments of theinvention in which an exclusive property or,privilege is claimed are defined a's follows:

l. A capacitiveposition sensor for indicating the position of arotatable member comprising, in combination, p

a rotatable, radially elongated capacitor plate operatively connected tosaid member for rotation therewith, I

v aplurality of stationary capacitor plates circumferentially displacedfrom each other in a plane adjacent to said rotatable plateandperpendicular to the axis thereof and each of which subtendsasubstantially greater arc aboutsaid axis of rotation than said rotatableplate, an oscillator adapted to generate high frequency pulses, 1 meansfor couplingthe output of said oscillator to said rotatable-plate, and

indicating means coupled to said stationary capacitor plates andresponsive to the high frequency pulses from said oscillator passing.through the capacitance between said rotatable plate and each saidstationary plate as said rotatable plate revolves for generating anoutput pulse when said rotatable plate is opposite each stationary plateindicative of the instantaneous position of said rotatable member.

2. A capacitive position sensor in accordance with claim 1 wherein saidindicating means includes a plurality of differential amplifiers eachhaving its inputs coupled'to apair of stationary capacitor platesdisplaced 180 electrical degrees apart, whereby a logic one voltagesignal is coupled to one of said inputs from the stationary plate ofsaid pair opposite said rotatable plate at a given instant and a logicaero voltage signal is simultaneously coupled to the other input fromthe other stationary plate of said pair.

3. A capacitive position sensor in accordance with claim 2 wherein saidindicating means includes a plurality of bistable circuit means each ofwhich receives the output from one of said differential amplifiers forderiving square wave pulses indicative of the position of said rotatablemember, said bistable circuit means providing positive feedback to theinputs of said differential amplifier to enhance discrimination betweenstationary capacitor plates.

4. A capacitive position sensor in accordance with claim 11 wherein saidoscillator is adapted to generate fast-rise pulses and has a frequencysubstantially higher than the rpm. of said rotatable member.

5. A capacitive position sensor in accordance with claim 1 forgenerating an n-phase train of output pulses indicative of the positionof said rotatable member and at a frequency proportional to the angularvelocity of said rotatable member, where'n is an integer, and whereinsaid indicating means includes n pulse amplifier meanseach of which iselectrically coupled to a pluralityof adjacent first stationarycapacitor plates tionary plates coupled to said which together subtendelectrical degrees and are displaced 360/71 electrical degrees from thefirst stationary plates "coupled to the other pulse amplifier means.

6. A capacitive position sensor in accordance with claim 1 forgenerating an n-phase train of output pulses indicative of the positionof said rotatable member and at a frequency which is a function of theangular velocity of said rotatable member, where n is an integer, andwherein said indicating means includes n differential amplifiers each ofwhich has one of its inputs electrically coupled to a plurality ofadjacent first stationary capacitor plates which together subtend 180electrical degrees and its other input coupled to a plurality ofadjacent second stationary capacitor plates which are displaced 180electrical degrees from said first stationary plates, and whereinsaidfirst stationary capacitor plates coupled to said one input of eachdifferential amplifier are displaced 360/n electrical degrees from thefirst sta one input of each of the other differential amplifiers: f

7. A capacitive'position sensor in accordance with claim I wherein saidrotatable member is the rotor of an electric motor having n phases and protor pole pairs, where n and p are integers, and wherein said rotorposition sensor includes at least n times p of said stationary capacitorplates and derives an n-phase train of output pulses indicative of theinstantaneous position of said rotor having a frequency which is afunction of the angular velocity of said rotor. I

8. A capacitive rotor position sensor in accordance with claim 7including n times 2p stationary capacitor plates and said indicatingmeans includes n differential amplifiers each of which derivesthe-output pulses forone of said phases and has its inputs coupled tostationary capacitor plates displaced 180 electrical degrees apart.

9. A capacitive rotor position sensor in accordance with claim 8 whereina plurality of first stationary capacitor plates which together subtend180 electrical degrees are electrically coupled to one input of each ofsaid differential amplifiers and are displaced 360/n electrical degreesfrom the first stationary capacitor plates which are coupled to said oneinput of each of the other differential amplifiers, whereby each saiddifferential amplifier derives output pulses of 180 electrical degreesduration for one phase displaced 360/n electrical degrees from the otherphase output pulses.

10. A capacitive rotor position sensor in accordance with claim 9wherein a plurality of second stationary capacitor plates which togethersubtend 180 electrical degrees are electrically coupled to the otherinput of each of said differential amplifiers and are displaced 180electrical degrees from said first stationary plates which are coupledto said one input, and wherein each of said first and said secondstationary plates subtend 360/2n electrical degrees, whereby a phaseoutput pulse of 180 electrical degrees duration is generated when saidrotatable plate is opposite said first stationary plates and itscomplement is generated when said rotatable plate is opposite saidsecond stationary plates.

11. A capacitive rotor position sensor in accordance with claim 7including n times 2p stationary capacitor plates and said indicatingmeans includes 11 pulse amplifier means each of which is associated withone phase of said motor and is electrically coupled to a plurality offirst stationary capacitor plates each of which subtends 360/2nelectrical degrees and which together subtend 180 electrical degrees andwhich are displaced 360/n electrical degrees from the first stationaryplates coupled to each of the other pulse amplifying means.

12. A capacitive rotor position sensor in accordance with claim 9wherein p is equal to at least two and said rotatable plate has aradially elongated arm for each pole pair and said radially elongatedarms are disposed 360 electrical degrees apart and also whereinstationary plates displaced 360 electrical degrees apart are associatedwith the same phase and are coupled to the same differential amplifier,whereby the signals from said stationary capacitor plates displaced 360electrical degrees apart are additive.

l3 .A capacitive position sensor in accordance with claim 1 andincluding grounded conductor means extending radially between adjacentstationary capacitor plates for electrically isolating them from eachother.

14. A capacitive position sensor in accordance with claim 1 andincluding a coupling capacitor having a stationary electrode disposed insaid plane radially inward from said stationary capacitor platesyand arotatable electrode spaced by an air gap in an axial direction from saidstationary electrode and having a radially extending portion integraltherewith opposite said stationary capacitor plates constituting saidrotatable capacitor plate, said oscillator being coupled to saidrotatable plate through said coupling capacitor.

15. A capacitive rotor position sensor in accordance with claim 10 andincluding grounded conductor means extending radially between adjacentstationary capacitor plates for electrically isolating them from eachother.

16. A capacitive rotor position sensor in accordance with claim 10 andincluding a coupling capacitor having a stationary electrode disposed insaid plane radially inward f rom said stationary capacitor plates and arotatable electrode spaced by an air gap in an axial direction from saidstationary electrode and having a narrow radially extending portionintegral therewith constituting said rotatable capacitor plate, saidoscillator being coupled to said rotatable capacitive plate through saidcoupling capacitor.

17. A capacitive rotor position sensor for generating an n-phase trainof pulses indicative of the position of the rotor of an electric motorhaving 11 phases and p pairs of rotor poles and at a frequencyproportional to the angular velocity of said rotor, where n and p areintegers, comprising, in combination,

a rotatable, radially elongated capacitor plate operatively connected tosaid rotor for rotation therewith,

2pn stationary capacitor plates each of which subtends an arc ofapproximately 360/2n electrical degrees disposed adjacent said rotatableplate in a plane perpendicular to the axis of rotation of said rotatableplate and being circumferentially displaced from each other in saidplane,

a high frequency oscillator adapted to generate steep wavefront pulseselectrically coupled to said rotatable plate, and

n indicating means each associated with one phase of said motor andincluding a load impedance coupled to a plurality of adjacent firststationary capacitor plates for generating an output pulse in responseto the high frequency pulses from said oscillator coupled through thecapacitance between said rotatable and stationary plates when saidrotatable plate is opposite each of said stationary plates as itrevolves, the plurality of adjacent first stationary plates of each saidindicating means coupled to said load impedance of one phase togethersubtending electrical degrees and being displaced 360/n electricaldegrees from the first stationary plates coupled to the load impedancesassociated with the other phases.

18. A capacitive rotor position sensor in accordance with claim 17wherein each said indicating means includes a differential amplifierhaving one input coupled to said load impedance and to said firststationary plates of the associated phase and having its other inputcoupled to a plurality of adjacent second stationary plates whichtogether subtend 180 electrical degrees and are displaced 180 electricaldegrees from said first plates.

19. A capacitive rotor position sensor in accordance with claim 17 andincluding grounded conductor means extending radially between adjacentstationary plates for electrically isolating them from each other.

20. A capacitive rotor position sensor in accordance with claim 18wherein each said indicating means also includes bistable circuit meanscoupled to the output of the associated differential amplifier forgenerating square wave pulses indicative of the position of said rotor,said bistable circuit means providing positive feedback to the inputs tothe associated differential amplifier to enhance discrimination betweenstationary capacitor plates.

21. A capacitive rotor position sensor in accordance with claim 17 andincluding a coupling capacitor having a stationary electrode disposed insaid plane radially inward from said stationary capacitor plates and arotatable electrode spaced by an air gap in an axial direction from saidstationary electrode and having a narrow radially extending portionintegral therewith constituting said rotatable capacitor plate, saidoscillator being coupled to said rotatable capacitive plate through saidcoupling capacitor. I

22. A capacitor rotor position sensor in accordance with claim 17wherein p is equal toat least two and said rotatable capacitor plate hasa radially elongated arm for each pole pair and said radially elongatedarms are disposed 360 electrical degrees apart, and also whereinstationary capacitor plates displaced 360 electrical degrees apart areassociated with the same phase and are coupled to the same differentialamplifier, whereby the signals from said stationary plates displaced 360electrical degrees apart are additive.

23. A capacitive rotor position sensor for generating a three-phasetrain of pulses indicative of the position of the rotor of an electricmotor having three pairs of rotor poles comprising, in combination,

a rotatable, diametrically elongated capacitor plate operativelyconnected to said rotor for rotation therewith,

1 2 stationary capacitor plates circumferentially displaced fromeachother in a plane adjacent to said rotatable plate and perpendicularto the axis of rotation thereof, each of said stationaryplates being inthe shape of a sector of a ring and subtending an arc of approximately30,

a high frequency oscillator coupled to said rotatable plate, and

three differential amplifiers, one input of each of said 24. Acapacitive rotor position sensor in accordance with claim 23 andincluding,

a coupling capacitor having a stationary electrode disposed in saidplane radially inward from said sta' tionary capacitor plates and arotatable electrode spaced in an axial direction by an air gap from saidstationary electrode and having a pair of diametrically opposed,radially extending, narrow portions integral therewith positionedopposite said stationary capacitor plates and together constituting saidrotatable capacitor plate, said oscillator being coupled to saidrotatable capacitor plate by said coupling capacitor. f V

25. A capacitive rotor position sensor in accordance with claim 24 andincluding grounded conductor means extending radially between adjacentstationary plates and also between the inner periphery of saidstationary plates and said coupling'capacitor stationary electrode forelectrically isolating said stationary capacitor plates from each other.

1. A capacitive position sensor for indicating the position of arotatable member comprising, in combination, a rotatable, radiallyelongated capacitor plate operatively connected to said member forrotation therewith, a plurality of stationary capacitor platescircumferentially displaced from each other in a plane adjacent to saidrotatable plate and perpendicular to the axis thereof and each of whichsubtends a substantially greater arc about said axis of rotation thansaid rotatable plate, an oscillator adapted to generate high frequencypulses, means for coupling the output of said oscillator to saidrotatable plate, and indicating means coupled to said stationarycapacitor plates and responsive to the high frequency pulses from saidoscillator passing through the capacitance between said rotatable plateand each said stationary plate as said rotatable plate revolves forgenerating an output pulse when said rotatable plate is opposite eachstationary plate indicative of the instantaneous position of saidrotatable member.
 2. A capacitive position sensor in accordance withclaim 1 wherein said indicating means includes a plurality ofdifferential amplifiers each having its inputs coupled to a pair ofstationary capacitor plates displaced 180 electrical degrees apart,whereby a logic one voltage signal is coupled to one of said inputs fromthe stationary plate of said pair opposite said rotatable plate at agiven instant and a logic zero voltage signal is simultaneously coupledto the other input from the other stationary plate of said pair.
 3. Acapacitive position sensor in accordance with claim 2 wherein saidindicating means includes a plurality of bistable circuit means each ofwhich receives the output from one of said differential amplifiers forderiving square wave pulses indicative of the position of said rotatablemember, said bistable circuit means providing positive feedback to theinputs of said differential amplifier to enhance discrimination betweenstationary capacitor plates.
 4. A capacitive position sensor inaccordance with claim 1 wherein said oscillator is adapted to generatefast-rise pulses and has a frequency substantially higher than ther.p.m. of said rotatable member.
 5. A capacitive position sensor inaccordance with claim 1 for generating an n-phase train of output pulsesindicative of the position of said rotatable member and at a frequencyproportional to the angular velocity of said rotatable member, where nis an integer, and wherein said indicating means includes n pulseamplifier means each of which is electrically coupled to a plurality ofadjacent first stationary capacitor plates which together subtend 180electrical degrees and are displaced 360/n electrical degrees from thefirst stationary plates coupled to the other pulse amplifier means.
 6. Acapacitive position sensor in accordance with claim 1 for generating ann-phase train oF output pulses indicative of the position of saidrotatable member and at a frequency which is a function of the angularvelocity of said rotatable member, where n is an integer, and whereinsaid indicating means includes n differential amplifiers each of whichhas one of its inputs electrically coupled to a plurality of adjacentfirst stationary capacitor plates which together subtend 180 electricaldegrees and its other input coupled to a plurality of adjacent secondstationary capacitor plates which are displaced 180 electrical degreesfrom said first stationary plates, and wherein said first stationarycapacitor plates coupled to said one input of each differentialamplifier are displaced 360/n electrical degrees from the firststationary plates coupled to said one input of each of the otherdifferential amplifiers.
 7. A capacitive position sensor in accordancewith claim 1 wherein said rotatable member is the rotor of an electricmotor having n phases and p rotor pole pairs, where n and p areintegers, and wherein said rotor position sensor includes at least ntimes p of said stationary capacitor plates and derives an n-phase trainof output pulses indicative of the instantaneous position of said rotorhaving a frequency which is a function of the angular velocity of saidrotor.
 8. A capacitive rotor position sensor in accordance with claim 7including n times 2p stationary capacitor plates and said indicatingmeans includes n differential amplifiers each of which derives theoutput pulses for one of said phases and has its inputs coupled tostationary capacitor plates displaced 180 electrical degrees apart.
 9. Acapacitive rotor position sensor in accordance with claim 8 wherein aplurality of first stationary capacitor plates which together subtend180 electrical degrees are electrically coupled to one input of each ofsaid differential amplifiers and are displaced 360/n electrical degreesfrom the first stationary capacitor plates which are coupled to said oneinput of each of the other differential amplifiers, whereby each saiddifferential amplifier derives output pulses of 180 electrical degreesduration for one phase displaced 360/n electrical degrees from the otherphase output pulses.
 10. A capacitive rotor position sensor inaccordance with claim 9 wherein a plurality of second stationarycapacitor plates which together subtend 180 electrical degrees areelectrically coupled to the other input of each of said differentialamplifiers and are displaced 180 electrical degrees from said firststationary plates which are coupled to said one input, and wherein eachof said first and said second stationary plates subtend 360/2nelectrical degrees, whereby a phase output pulse of 180 electricaldegrees duration is generated when said rotatable plate is opposite saidfirst stationary plates and its complement is generated when saidrotatable plate is opposite said second stationary plates.
 11. Acapacitive rotor position sensor in accordance with claim 7 including ntimes 2p stationary capacitor plates and said indicating means includesn pulse amplifier means each of which is associated with one phase ofsaid motor and is electrically coupled to a plurality of firststationary capacitor plates each of which subtends 360/2n electricaldegrees and which together subtend 180 electrical degrees and which aredisplaced 360/n electrical degrees from the first stationary platescoupled to each of the other pulse amplifying means.
 12. A capacitiverotor position sensor in accordance with claim 9 wherein p is equal toat least two and said rotatable plate has a radially elongated arm foreach pole pair and said radially elongated arms are disposed 360electrical degrees apart and also wherein stationary plates displaced360 electrical degrees apart are Associated with the same phase and arecoupled to the same differential amplifier, whereby the signals fromsaid stationary capacitor plates displaced 360 electrical degrees apartare additive.
 13. A capacitive position sensor in accordance with claim1 and including grounded conductor means extending radially betweenadjacent stationary capacitor plates for electrically isolating themfrom each other.
 14. A capacitive position sensor in accordance withclaim 1 and including a coupling capacitor having a stationary electrodedisposed in said plane radially inward from said stationary capacitorplates, and a rotatable electrode spaced by an air gap in an axialdirection from said stationary electrode and having a radially extendingportion integral therewith opposite said stationary capacitor platesconstituting said rotatable capacitor plate, said oscillator beingcoupled to said rotatable plate through said coupling capacitor.
 15. Acapacitive rotor position sensor in accordance with claim 10 andincluding grounded conductor means extending radially between adjacentstationary capacitor plates for electrically isolating them from eachother.
 16. A capacitive rotor position sensor in accordance with claim10 and including a coupling capacitor having a stationary electrodedisposed in said plane radially inward from said stationary capacitorplates and a rotatable electrode spaced by an air gap in an axialdirection from said stationary electrode and having a narrow radiallyextending portion integral therewith constituting said rotatablecapacitor plate, said oscillator being coupled to said rotatablecapacitive plate through said coupling capacitor.
 17. A capacitive rotorposition sensor for generating an n-phase train of pulses indicative ofthe position of the rotor of an electric motor having n phases and ppairs of rotor poles and at a frequency proportional to the angularvelocity of said rotor, where n and p are integers, comprising, incombination, a rotatable, radially elongated capacitor plate operativelyconnected to said rotor for rotation therewith, 2pn stationary capacitorplates each of which subtends an arc of approximately 360/2n electricaldegrees disposed adjacent said rotatable plate in a plane perpendicularto the axis of rotation of said rotatable plate and beingcircumferentially displaced from each other in said plane, a highfrequency oscillator adapted to generate steep wavefront pulseselectrically coupled to said rotatable plate, and n indicating meanseach associated with one phase of said motor and including a loadimpedance coupled to a plurality of adjacent first stationary capacitorplates for generating an output pulse in response to the high frequencypulses from said oscillator coupled through the capacitance between saidrotatable and stationary plates when said rotatable plate is oppositeeach of said stationary plates as it revolves, the plurality of adjacentfirst stationary plates of each said indicating means coupled to saidload impedance of one phase together subtending 180 electrical degreesand being displaced 360/n electrical degrees from the first stationaryplates coupled to the load impedances associated with the other phases.18. A capacitive rotor position sensor in accordance with claim 17wherein each said indicating means includes a differential amplifierhaving one input coupled to said load impedance and to said firststationary plates of the associated phase and having its other inputcoupled to a plurality of adjacent second stationary plates whichtogether subtend 180 electrical degrees and are displaced 180 electricaldegrees from said first plates.
 19. A capacitive rotor position sensorin accordance with claim 17 and including grounded conductor meansextending radially between adjacent stationary plates for electricallyisolating them from each other.
 20. A capacitive rotor position sensorin accordance with claim 18 wherein each said indicating means alsoincludes bistable circuit means coupled to the output of the associateddifferential amplifier for generating square wave pulses indicative ofthe position of said rotor, said bistable circuit means providingpositive feedback to the inputs to the associated differential amplifierto enhance discrimination between stationary capacitor plates.
 21. Acapacitive rotor position sensor in accordance with claim 17 andincluding a coupling capacitor having a stationary electrode disposed insaid plane radially inward from said stationary capacitor plates and arotatable electrode spaced by an air gap in an axial direction from saidstationary electrode and having a narrow radially extending portionintegral therewith constituting said rotatable capacitor plate, saidoscillator being coupled to said rotatable capacitive plate through saidcoupling capacitor.
 22. A capacitor rotor position sensor in accordancewith claim 17 wherein p is equal to at least two and said rotatablecapacitor plate has a radially elongated arm for each pole pair and saidradially elongated arms are disposed 360 electrical degrees apart, andalso wherein stationary capacitor plates displaced 360 electricaldegrees apart are associated with the same phase and are coupled to thesame differential amplifier, whereby the signals from said stationaryplates displaced 360 electrical degrees apart are additive.
 23. Acapacitive rotor position sensor for generating a three-phase train ofpulses indicative of the position of the rotor of an electric motorhaving three pairs of rotor poles comprising, in combination, arotatable, diametrically elongated capacitor plate operatively connectedto said rotor for rotation therewith, 12 stationary capacitor platescircumferentially displaced from each other in a plane adjacent to saidrotatable plate and perpendicular to the axis of rotation thereof, eachof said stationary plates being in the shape of a sector of a ring andsubtending an arc of approximately 30*, a high frequency oscillatorcoupled to said rotatable plate, and three differential amplifiers, oneinput of each of said differential amplifiers being coupled to threeadjacent first stationary plates together subtending approximately 90*and to the three stationary plates diametrically opposed thereto, andthe other input being coupled to three adjacent second stationary platescircumferentially displaced 90* from said first stationary plates andalso to the stationary plates diametrically opposed to said secondstationary plates, the corresponding inputs of said three differentialamplifiers being coupled to stationary plates displaced 120* apart. 24.A capacitive rotor position sensor in accordance with claim 23 andincluding, a coupling capacitor having a stationary electrode disposedin said plane radially inward from said stationary capacitor plates anda rotatable electrode spaced in an axial direction by an air gap fromsaid stationary electrode and having a pair of diametrically opposed,radially extending, narrow portions integral therewith positionedopposite said stationary capacitor plates and together constituting saidrotatable capacitor plate, said oscillator being coupled to saidrotatable capacitor plate by said coupling capacitor.
 25. A capacitiverotor position sensor in accordance with claim 24 and including groundedconductor means extending radially between adjacent stationary platesand also between the inner periphery of said stationary plates and saidcoupling capacitor stationary electrode for electrically isolating saidstationary capacitor plates from each other.